Systems and methods herein generally relate to printing devices, and more particularly to the coordination of the printhead/substrate position with the transfer of marking material from the printhead to the substrate.
In printing devices, it can be difficult to accurately register drops in the process direction, and ensure that drops from separated heads are printed at the required absolute location, especially for 3-dimensional printing. Printheads, such as inkjet printheads, fire when they receive a signal, such as a dot clock signal to cause marking material to be applied to a substrate, such as print media, a plate or platform, etc., to produce printed media, form 3-D items, etc.
Ink-jet printers fire drops of ink from the head in response to a clock signal. The clock signal is generated based on encoder feedback. Frequently drive-shaft mounted encoders are not perfectly co-axial, leading to a sinusoidal runout error in encoder spacing. For a continuous feed press, this can be corrected by generating a runout correction table by printing extremely long (20 m) test patterns and analyzing them with an image sensor. For a cut-sheet or 3D printing system, printing long test patterns is not possible.
This disclosure uses the measured time (for example, a number of 100 MHz clock counts) between encoder tics to measure runout and generate runout correction tables/functions to accurately calculate the positions of the media transport as a function of an angular position of the encoder roll. The time between tics is a strong function of the transport velocity, however averaging over many encoder roll revolutions allows a correction to be calculated accurately.